Heat exchangers have long been used to raise or lower the temperature of a working fluid. Several basic designs accomplish this end, but invariably each relies on the basic principle of thermodynamics that thermal energy will tend to migrate from a warm body to a cooler one. One common type of heat exchanger circulates the working fluid through a tube which is immersed in a bath of coolant contained within a casing. Thus the thermal energy will pass from the hotter of the two fluids, through the walls of the tube, to the cooler fluid. The rate of energy transfer is the greatest where the temperature gradient is large, and decreases as the temperature of both fluids approaches equilibrium.
Since the thermal energy transfer between the fluids increases as the surface area of the tube increases, the tube is ideally wound into a coil or otherwise condensed in size to maximize the surface area exposed to the fluids while minimizing the size of the casing. Moreover, in order to maintain continuous operation, fresh coolant is preferably circulated through the casing.
One particularly efficient design that incorporates both of these features is described in U.S. Pat. No. 3,526,273, to Wentworth (the "Wentworth patent"), which is incorporated herein by reference. This patent describes a heat exchanger in which the casing defines a cylindrical annular space, and the tube is wrapped into a helical coil which fits inside the annular space. The bottom end of the annular space is closed by an endwall, and on the top end there is a detachable cover. Both the inlet and outlet ends of the tube extend through the cover, and the coil is wrapped into multiple overlapping layers that spiral alternatingly between the endwall and the cover. Coolant is introduced into the casing through a second set of ports in the cover, and circulates around the outside of the tube in a spiral path corresponding to the turns of the helical coil. By forcing the coolant to travel along the path of the spiraling tube, heat transfer between the fluids is maximized. Also since the cover is detachable, the helical coil may be pulled from the annular space for maintenance and/or cleaning.
While the above described heat exchanger functions quite well, it does have its disadvantages. One disadvantage of this earlier design is the difficulty of venting and draining the tube coil. Venting of entrapped gasses inside the coil is very important because without proper venting these gases can severely impede the flow of fluid within the coil. This results in ineffective cooling or stalled flow, which can cause severe overheating. Venting of the coil to remove entrapped gases is difficult unless the heat exchanger is mounted in a vertical upright position with inlet and outlet fittings on top. However, when placed in this vertical position the coil cannot be drained. If the heat exchanger is placed on it's side (axis placed horizontal to the ground) both venting and draining become very difficult. Also, when the heat exchanger is placed on its side, sediment settles on the bottom of the casing obstructing the flow of coolant.
The earlier design has another disadvantage in that both the working fluid and the coolant flow down the case through one layer of the coil and back up the case through the adjacent layer of the coil. This double pass flow design increases the dwell time during which the coolant remains in the heat exchanger and results in an increased rise of temperature of the coolant.
An additional disadvantage is the difficulty of removing the coil from the casing for cleaning. The coolant (usually water) is in direct contact with the coil as well as the casing walls. Thus any impurities from the coolant, as well as any corrosion of the casing walls and tubes caused by the coolant, will eventually build-up restricting coolant flow and decrease the interval period between cleanings. To the extent that this build-up creates a bond between the coil and the casing, it becomes increasingly difficult, if not impossible, to remove the coil assembly from the casing without severe deformation to the coil in order to accomplish cleanings. In particular, build-ups are also an increasing problem due to increasing environmental restrictions on chemical treatment of cooling water to remove impurities.
When the coil assembly is to be removed from the casing it must be pulled from the open top end. This removal process almost invariably results in stretching of the coil, making reassembly difficult. In cases of severe build-up the coil will most likely be damaged when removed and the coil, and possibly the heat exchanger, will have to be replaced.
Another type of heat exchanger is described in U.S. Pat. No. 3,803,499 to Garcea. This heat exchanger discloses one finned tube formed into a single helical coil which passes through the casing, wherein coolant flows, and allows the working fluid to make one pass through the casing. A tie-rod passes through the axial bore of the heat exchanger to hold the end covers in place and thus secures the components of the heat exchanger.
A disadvantage of this design is that it discloses only one tube. By only using one tube the amount of working fluid per interval of time that passes through the casing is limited. An additional disadvantage is the difficulty of removing the coil from the casing for cleaning. The coolant is in direct contact with the coil as well as the casing walls, thus impurities can build-up between the casing walls and the coils creating many problems including increased difficulty in removal of the coils for cleaning. Furthermore, there appear to be supports that extend radially outward from the top and bottom of the inner cylindrical wall that extend partially around the top and bottom convolutions of the helical coil which would also restrict removal of the coil from the casing.
A combined heat exchanger and homogenizer titled "Device for Preparing Putty and Similar Masses" is described in U.S. Pat. No. 5,046,548 to Tilly. This patent discloses dual helical tubes within a casing, an additional tube located along the axial bore of the heat exchanger, and end plates. This device heats and homogenizes viscous masses, particularly putty. The putty passes through the casing under pressure and heating. The dual helical tubes and the additional tube located along the axial bore of the heat exchanger act as guiding devices to force the putty into a plurality of directional changes.
The above-described heat exchanger and homogenizer has many disadvantages in terms of operation as a conventional heat exchanger. One disadvantage is that it includes a straight heat exchanger tube located along the axial bore of the heat exchanger which extends through both the bottom and top end plates. As mentioned previously, tubes are ideally wound into a coil to maximize the surface area exposed to the fluids while minimizing the size of the case. The straight heat exchanger is very inefficient for the purposes of heat transfer and further is an inefficient use of space.
An additional disadvantage of this structure is that the dual helical coils are not sandwiched between an inner casing and an outer casing. The dual helical coils are instead arranged around and spaced from a straight heat exchanger tube located along the axial bore of the heat exchanger which extends through both the bottom and top end plates. As a result, flow through the casing is not adequately restricted nor channeled sufficiently over the dual helical coils. Therefore coolant will not be forced over the coils adequately nor will the coolant spiral satisfactorily over the coils.
A further disadvantage of this structure is that it only has a single chamber through which fluid may flow. The single chamber contains dual helical coils and an additional tube located along the axial bore of the single chamber which act as guiding devices to force viscous masses, particularly putty, into a plurality of directional changes. While the single chamber is apparently useful for homogenizing putty, it is inadequate for channeling fluid flow sufficiently over each individual coil in isolation from the other coil. Therefore coolant will not be restricted to flow through a separate chamber containing an individual coil and thus will not flow and spiral adequately over each individual coil. This results in an inefficient method of heat transfer between each individual coil and the coolant.
In view of the above, it should be appreciated that there is a need for an improved heat exchanger that provides the advantages of having a multiple tube helical coil configuration arranged within a shell assembly which allows differing flow patterns for working fluids, permits simplified venting and draining, allows coolant to pass through the shell assembly in a single pass flow through design, prevents the significant build-up of impurities or corrosive bonding between the multiple coiled tubes and the casing walls due to the circulation of coolant, channels coolant efficiently over the multiple coiled tubes, and enables easy removal of the multiple coiled tubes from the shell assembly for periodic cleaning or maintenance with little or no damage. The present invention satisfies these and other needs and provides further related advantages.